Both right, both wrong. The psu top mounts in the M11 case. It's a standard ATX psu. However, there are some restrictions placed with top mount psus. It has to be short. You can't just use ANY ATX psu since there's optical bays directly in front of it, so longer psus (usually the large wattage kind) simply do not fit, either physically or the wiring harness is a major obstacle. So, you'll need the short ATX, like the 140mm Seasonic's or Corsair CX/CXM. Then there's also physical differences in the case itself. Many of those top mount psus have a bracket with a tab attached that goes into a slot on the underside of the psu. Helps hold up the weight of the psu by the wiring harness. Aftermarket psus do not have this slot, so you'll need to modify the case by removal of that tab or flattening it to allow the psu to bolt up correctly and without pressure.
So, Asus support was probably correct, they probably knew of some obstruction and cannot advise you to tamper with the case (does things like voids warrantys and/or gets them fired for specifically telling you it'll work when it doesn't), and animemangamer was partially correct when he claims any will work, because they do work on the motherboard, just might not work for the case. Do not take his advice though, the Evga W1 and Corsair VS are not what I'd call decent, the only psu worse than the W1 is the N1, so basically both are at the very bottom of the psu ladder. Not good quality by a long shot, especially when paired with a dedicated gpu.
Psu wattage depends on the gpu. You can only fit the physically smaller gpus into that case, so a 550w psu is about as much as needed to cover anything. Again, the Corsair CX/CXM or TXM, Seasonic S12-II/M12-II 520w/620w or Focus would be good choices.